Production of fine-grained particles

ABSTRACT

Particles of mixed metal oxide include at least two metal species. The particles have a grain size within the range of 1-100 nm. The particles are substantially crystalline. The particles contain only small or negligible amounts of amorphous material. The at least two metal species are uniformly dispersed in the particles.

FIELD OF THE INVENTION

The present invention relates to very fine-grained particulate materialand to methods for producing such very fine-grained particulatematerial. In preferred aspects, the present invention relates to oxidematerials of very fine-grained particulate material and to methods forproducing such material. Most suitably, the particulate material hasgrain sizes in the nanometre scale.

BACKGROUND OF THE INVENTION

Metal oxides are used in a wide range of applications. For example,metal oxides can be used in:

-   -   solid oxide fuel cells (in the cathode, anode, electrolyte and        interconnect);    -   catalytic materials (automobile exhausts, emission control,        chemical synthesis, oil refinery, waste management);    -   magnetic materials;    -   superconducting ceramics;    -   optoelectric materials;    -   sensors (eg gas sensors, fuel control for engines);    -   structural ceramics (eg artificial joints).

Conventional metal oxides typically have grain sizes that fall withinthe micrometre range and often are supplied in the form of particleshaving particle sizes greater than the micrometre range. It is believedthat metal oxides that are comprised of nanometre sized grains will haveimportant advantages over conventional metal oxides. These advantagesinclude lower sintering temperatures, potentially very high surfaceareas, and sometimes improved or unusual physical properties. However,the ability to economically produce useful metal oxide materials withnanometre-sized grains has proven to be a major challenge to materialsscience. It has proven to be difficult to make such fine-scale metaloxides, particularly multi-component metal oxides, with:

-   -   (a) the correct chemical composition;    -   (b) a uniform distribution of different atomic species;    -   (c) the correct crystal structure; and    -   (d) a low cost.

Many important metal oxides have not yet been produced with very finegrains, especially multi-component metal oxides. This is because as thenumber of different elements in an oxide increases, it becomes moredifficult to uniformly disperse the different elements at the ultra-finescales required for nanometre-sized grains. A literature searchconducted by the present inventors has shown that very small grain sizes(less than 20 nm) have only been attained for a limited number of metaloxides. The reported processes used to achieve fine grain size are veryexpensive, have low yields and can be difficult to scale up. Many of thefine grained materials that have been produced do not displayparticularly high surface areas, indicating poor packing of grains.

At this stage, it will be realised that particles of material aretypically agglomerates of a number of grains. Each grain may be thoughtof as a region of distinct crystallinity joined to other grains. Thegrains may have grain boundaries that are adjacent to other grainboundaries. Alternatively, some of the grains may be surrounded by andagglomerated with other grains by regions having a different composition(for example, a metal, alloy or amorphous material) to the grains.

Methods described in the prior art for synthesising nano materialsinclude gas phase synthesis, ball milling, co-precipitation, sol gel,and micro emulsion methods. The methods are typically applicable todifferent groups of materials, such as metals, alloys, intermetallics,oxides and non-oxides. A brief discussion of each will follow:

Gas-Phase Synthesis

Several methods exist for the synthesis of nano-particles in the gasphase. These include Gas Condensation Processing, Chemical VapourCondensation, Microwave Plasma Processing and Combustion Flame Synthesis(H. Hahn, “Gas Phase Synthesis of Nanocrystalline Materials”, NanoStructured Materials, Vol 9, pp 3-12, 1997). In these methods thestarting materials (suitable precursors to a metal, alloy or aninorganic material) are vaporised using energy sources such as Jouleheated refractory crucibles, electron beam evaporation devices,sputtering sources, hot wall reactors, etc. Nano-sized clusters are thencondensed from the vapour in the vicinity of the source by homogenousnucleation. The clusters are subsequently collected using a mechanicalfilter or a cold finger. These methods produce small amounts ofnon-agglomerated material, with a few tens of gram/hour quoted as asignificant achievement in production rate.

Ball Milling

Mechanical attrition or ball milling is another method that can be usedto produce nano-crystalline materials (C. C. Koch, “Synthesis ofNanostructured Materials by Mechanical Milling: Problems andOpportunities”, Nano Structured Materials, Vol 9, pp 13-22, 1997).Unlike the aforementioned methods, mechanical attrition produces thenano-materials not by cluster assembly but by the structuraldecomposition of coarser-grained materials as a result of severe plasticdeformation. The quality of the final product is a function of themilling energy, time and temperature. To achieve grain sizes of a fewnanometres in diameter requires relatively long processing times(several hours for small batches). Another main drawback of the methodis that the milled material is prone to severe contamination from themilling media.

Co-Precipitation

In some special cases it is possible to produce nano-crystallinematerials by precipitation or co-precipitation if reaction conditionsand post-treatment conditions are carefully controlled (L. V. Interranteand M. J. Hampden-Smith), Chemistry of Advanced Materials—An Overview,Wiley—VCH (1998)). Precipitation reactions are among the most common andefficient types of chemical reactions used to produce inorganicmaterials at industrial scale. In a precipitation reaction, typically,two homogenous solutions are mixed and an insoluble substance (a solid)is subsequently formed. Conventionally, one solution is injected into atank of the second solution in order to induce precipitation, however,simultaneous injection of the two solutions is also possible. The solidthat forms (called the precipitate) can be recovered by methods such asfiltration.

The precursor material has subsequently to be calcined in order toobtain the final phase pure material. This requires, in particular,avoidance of phenomena that induce segregation of species duringprocessing such as partial melting for example. Formation of stableintermediates also has to be avoided since the transformation to thefinal phase pure material might become nearly impossible in that case.Typical results for surface areas for single oxides can be of severaltens of m²/g. However, for a multi-cation compound, values less than 10m²/g become more common.

Sol-gel Synthesis

Sol-gel synthesis is also a precipitation-based method. Particles orgels are formed by ‘hydrolysis-condensation reactions’, which involvefirst hydrolysis of a precursor, followed by polymerisation of thesehydrolysed percursors into particles or three-dimensional networks. Bycontrolling the hydrolysis-condensation reactions, particles with veryuniform size distributions can be precipitated. The disadvantages ofsol-gel methods are that the precursors can be expensive, carefulcontrol of the hydrolysis-condensation reactions is required, and thereactions can be slow.

Microemulsion Methods

Microemulsion methods create nanometre-sized particles by confininginorganic reactions to nanometre-sized aqueous domains, that existwithin an oil. These domains, called water-in-oil or inversemicroemulsions, can be created using certain surfactant/water/oilcombinations.

Nanometre-sized particles can be made by preparing two different inversemicroemulsions (eg (a) and (b)). Each microemulsion has a specificreactant dissolved in the aqueous domains. The inverse microemulsionsare mixed, and when the aqueous domains in (a) collide with those in(b), a reaction takes place that forms a particle. Since the reactionvolumes are small, the resultant particles are also small. Somemicroemulsion techniques are reviewed in “Nanoparticle and PolymerSynthesis in Microemulsion”, J. Eastoe and B. Wame, Current Opinion inColloid and Interface Science, vol. 1 (1996), p800-805, and “NanoscaleMagnetic Particles: Synthesis, Structure and Dynamics”, ibid, vol. 1(1996), p806-819.

A major problem with this technique is that the yield (wt product/wtsolution) is small. Most microemulsion systems contain less than ˜20 vol% aqueous domains, which reduces the yield from the aqueous phasereactions by a factor of ˜5. Many of the aqueous phase reactionsthemselves already have low yields, therefore a further significantreduction in yield is very undesirable. The method also requires removalof particles from the oil. This can be very difficult for nanosisedparticles surrounded by surfactant, since these particles can remainsuspended in solution, and are very difficult to filter due to theirsmall size. Once the particles are separated, residual oil andsurfactant still needs to be removed. Another serious disadvantage isthat reaction times can be quite long. These aspects together wouldgreatly increase the size, complexity and cost of any commercialproduction facility.

Use of Surfactants

Recently, there has been considerable research and development into theproduction of high surface area metal oxides using “surfactanttemplating”. Surfactants are organic (carbon-based) molecules. Themolecules have a hydrophilic (ie has an affinity for water) section anda hydrophobic (ie does not have an affinity for water) section.

Surfactants can form a variety of structures in aqueous (and other)solutions dependent upon the type of surfactant, the surfactantconcentration, temperature, ionic species, etc. The simplest arrangementis individual surfactant molecules dispersed in solution. This typicallyoccurs for very low concentration of surfactants. For higherconcentrations of surfactant, the surfactant can coalesce to form“micelles”. Micelles can be spherical or cylindrical. The diameter ofthe micelle is controlled mainly by the length of the surfactant chainand can range between ˜20 angstroms and ˜300 angstroms.

Even higher concentrations of surfactant give rise to more orderedstructures called “liquid crystals”. Liquid crystals consist of orderedmicelles (eg micellar cubic, hexagonal) or ordered arrays of surfactant(eg lamella, bicontinuous cubic), within a solvent, usually water.

A paper published by C T Kresge, M E Leonowicz, W J Roth, J C Vartuliand J S Beck, “Ordered Mesoporous Molecular Sieves Synthesized by aLiquid Crystal Template Mechanism”, Nature, vol 359 (1992) p710-712,described the production of inorganic materials having ordered porosity.In the process described in this paper, an ordered array of surfactantmolecules was used to provide a “template” for the formation of theinorganic material. The basic premise for this process was to use thesurfactant structures as a framework and deposit inorganic material ontoor around the surfactant structures. The surfactant is then removed(commonly by burning out or dissolution) to leave a porous network thatmimics the original surfactant structure. The process is shownschematically in FIG. 1. Since the diameter of the surfactant micellescan be extremely small, the pore sizes that can be created using themethod are also extremely small, and this leads to very high surfaceareas in the final product.

There are several characteristic features of the materials that havebeen produced using surfactant templating process as described above:

(a) An Ordered Pore Structure

As shown in FIG. 1, surfactant-templating methods use ordered surfactantstructures to template deposition of inorganic material. The surfactantis then removed without destroying the ordered structure. This resultsin an ordered pore network, which mimics the surfactant structure.

The size of the pores, the spacing between pores, and the type ofordered pore pattern are dependent upon the type of surfactant, theconcentration of the surfactant, temperature and other solutionvariables. Pores sizes between ˜20 angstroms and ˜300 angstroms havebeen achieved. Spacings between the pores also lie approximately withinthis range.

Periodic order at this scale can be detected using x-ray diffraction(XRD). In an XRD scan, signal intensity is plotted against the angle ofthe incident x-ray beam on the sample. Periodic structures give rise topeaks on XRD scans. The length of the periodic spacing is inverselyrelated to the angle at which the peak occurs. Periodic arrangements ofatoms (crystals), in which the spacings are very small, produce peaks atso-called ‘high angles’ (typically>5°). The ordered pore structures insurfactant-templated materials have much greater spacings, and thereforeproduce peaks at low angles (typically much less than 5°). A special XRDinstrument, called a small angle x-ray scattering (SAXS) instrument, iscommonly used to examine the pore structure in surfactant templatedmaterials. An example of an XRD scan from a surfactant-templatedmaterial is shown in FIG. 2.

(b) Uniform Pore Size

For a given type of surfactant, surfactant micelles are essentially thesame size. Pore sizes are therefore very uniform since pores are createdin the space that was occupied by the micelles. Pore size distributionsin materials may be obtained using nitrogen gas absorption instruments.An example of a pore size distribution from a surfactant-templatedmaterial is shown in FIG. 3. The distribution is extremely narrow, andis approximately centred on the diameter of the surfactant micelles.Such distributions are typical for surfactant templated materials.

(c) Absence of Atomic Crystallinity (ie. Absence of Highly OrderedAtomic Structures).

Most conventional inorganic materials are crystalline. That is, theiratoms are organised into highly ordered periodic structures. The type,amount and orientation of crystals in inorganic materials criticallyinfluences many important physical properties. A major drawback of mostsurfactant-templated materials is that normally the inorganic materialis not highly crystalline. In fact in most cases it is consideredamorphous.

The difficulties in producing highly crystalline materials derive fromrestrictions imposed by the very nature of surfactant templating. Theserestrictions greatly limit the types of reactions that can be used toform inorganic material. Obviously the inorganic material must formwhilst the surfactant structure is preserved. Since the surfactantstructure normally exists in an aqueous-based solution, the inorganicreactions must be aqueous-based, and must occur at temperatures lessthan 100° C. This restriction is severe. Many conventional metal oxidematerials, particularly complex multi-component oxides, require heattreatments at very high temperatures (up to 1200° C.) in order toachieve the correct crystal structure and a uniform dispersion ofelements.

(d) Long Reaction Times

Most surfactant-templating methods require long reaction times to formthe surfactant-inorganic structure. Following this, extended and carefulheat treatment is usually necessary to remove the surfactant. Longreaction times greatly add to the expense and inconvenience ofprocessing at a practical scale. The long reaction times again can beattributed to the types of inorganic reactions that must be employed insurfactant templating.

A variant on the surfactant templating method described above may bedescribed as the production of surfactant-templated structures via selfassembly. Many of the detailed mechanisms of this process are not clear,however the basic principle is that the surfactant-inorganic structuresassemble at a substrate or a nucleus and grow from there. A generalreview of this method is given by Aksay-IA; Trau-M; Manne-S; Honma-I;Yao-N; Zhou-L; Fenter-P; Eisenberger-PM; Grune-SM “Biomimetic pathwaysfor assembling inorganic thin films”, Science vol. 273 (1996), p892-898.

In self-assembly, the solution must be carefully controlled so thatinorganic deposition only occurs on the assembling surfactant structure.If the inorganic phase forms too rapidly, then large inorganicprecipitates that do not contain surfactant will form and drop out ofsolution. Clearly this would result in a non-porous structure.

The inorganic reactions that have mostly been employed in self-assembly(and other surfactant-templating methods as well) are called‘hydrolysis-condensation’ reactions. Hydrolysis-condensation reactionsinvolve an ‘inorganic precursor’, which is initially dissolved insolution. The first step in the reaction is hydrolysis of the precursor.This is followed by polymerisation of the hydrolysed precursor(condensation) to form an inorganic phase. Hydrolysis-condensationreactions may be represented generally as: M − OR + H₂O

M − OH + ROH hydrolysis M − OH + M − OR

M − O − M + ROH condensationM = a metal ionR = an organic ligand, e.g. CH₃M − OR = inorganic precursor, commonly an alkoxide

The polymerisation nature of these reactions results in glass-likematerials that do not contain a high degree of atomic order. Asdiscussed previously this is a major limitation of mostsurfactant-templated materials. It is possible to increase the order inthe inorganic material by heat treating at high temperatures, but almostall attempts to do this have resulted in collapse of the pore structureprior to crystallisation.

Most hydrolysis-condensation reactions are too rapid in aqueoussolutions to be useful for surfactant templating. Silica-based reactionsare an exception, and can be controlled very well. This explains why,for a long time, the only surfactant templated materials produced wereeither silica or silica-based.

Some success has been achieved with a number of other materials by usingadditives that slow down the hydrolysis condensation reactions inaqueous solutions. Examples are: “Synthesis of Hexagonal PackedMesoporous TiO₂ by a Modified sol-gel Method” Agnew. Chem. Int. EditionEnglish, vol. 34 (1995), p2014-2017, D. M. Antonelli and J. Y. Ying,ibid, vol. 35 (1996) p426, M. Froba, O. Muth and A. Reller,“Mesostructured TiO₂: Ligand-stabilised Synthesis and Characterisation”,Solid State Ionics, vols. 101-103 (1997), p249-253. A relevant patent isU.S. Pat. No. 5,958,367 (J. Y.Ying, D. M. Antonelli, T.Sun).

A major advance was accomplished by Stuckey et. al., (“GeneralisedSyntheses of Large-pore Mesoporous Metal Oxides with SemicrystallineFrame works”, P. Yang, D. Zhao, D. I. Margolese, B. F. Chmelka and G. D.Stucky, Nature, vol. 396 (1998), p152-155) who used alcohol-basedsolutions rather than aqueous solutions to form surfactant-templatedstructures. Hydrolysis-condensation reactions are much more easilycontrolled in alcohol solutions than aqueous solutions. Stucky et al.were therefore able to produce surfactant-templated structures with arange of inorganic metal oxides. Stucky, et. al. also reported thattheir materials exhibited some crystallinity in the organic phase.However the amount of crystallinity was still small, and the inorganicphase consisted of very small crystalline regions surrounded byamorphous inorganic material.

Surfactant-templated Structures via In-situ Reaction in Liquid Crystals

In this method, a solution of water and an inorganic precursor is mixedwith an appropriate amount of surfactant, and this mixture is kept at atemperature where the surfactant organises to form a liquid crystal. Theinorganic precursor then reacts to form inorganic material that occupiesthe space between the surfactant micelles. Finally the surfactant andany remaining water are removed by burning out or other methods.

Similar to the case for assembling surfactant structures, the inorganicreaction must take place while the surfactant structure is preserved.This again limits the temperature of the reaction, and the reaction musttake place in an aqueous solution. Also, the reaction should not proceedprior to, or during, mixing with the surfactant.

The majority of research has used the same silicatehydrolysis-condensation reactions described in the self-assembly method.The liquid crystal structure is retained in the final product, asevidenced either by small angle XRD peaks or TEM. High angle XRD peaks,which would indicate atomic crystalline structures, are not present.

A different reaction method has been employed to produce cadmiumsulfide, as outlined in “Semiconducting Superlattices templated byMolecular Assemblies”, P. Braum, P. Osenar and S. I. Stupp, Nature vol.380 (1996) p325-327, and “Countering Effects in Liquid CrystalTemplating of Nanostructured CdS”, V. Tohver et. al. Chemistry ofMaterials Vol 9, No. 7 (1997), p1495. Cadmium sulfate, cadmium chloride,cadmium perchlorate and cadmium nitrate aqueous solutions were mixedwith surfactants to create liquid crystals. H₂S gas was infused into thestructure, which reacted with the dissolved cadmium ions to produce CdS.The liquid Crystal structure is retained in final product. Importantly,significant high-angle x-ray peaks are present indicating good atomiccrystallinity.

Surfactant-templated Structures via Electrodeposition in Liquid Crystals

This method uses a similar principle to the surfactant-templatingmethods described above. An aqueous-based electroplating solution ismixed with surfactant at an appropriate concentration to form a liquidcrystal. This mixture is placed between two electrodes, and kept at atemperature where the surfactant organises to form a liquid crystal. Oneof the electrodes is a substrate that is to be coated. Applying anappropriate voltage causes inorganic material to be deposited at oneelectrode. This material only deposits in the space between thesurfactant. Upon completion of electrodeposition, the surfactant may beremoved by heating or by dissolution in a solvent that does not attackthe inorganic material.

The organised pore structure is maintained in this method. The depositedmaterial is almost always metal, which is very easy to crystallise,therefore strong high-angle XRD peaks are observed. Platinum and tinhave been produced by this technique.

As mentioned above, it is an aim of the surfactant-templating methodsdescribed above to produce solid material having a regular array ofpores, with the pore structure having a very narrow pore sizedistribution (i.e. the pores are essentially of the same diameter). Mostof the surfactant-templating processes described in the literature haveresulted in the formation of inorganic particles having a particle sizein excess of one micrometre. Crystallinity is difficult to obtain.Reaction times are lengthy because significant time is required to formthe surfactant-inorganic structure in solution. Indeed, a number ofpublished papers require time periods in the range of 1 day to 7 days toallow the desired surfactant-inorganic structure to develop.Furthermore, the conditions used to deposit the inorganic material inthe surfactant structure must be “gentle” in order to avoid collapse ofthe surfactant structure.

Another approach to producing nanopowders is described in U.S. Pat. No.5,698,483 to Ong et al. In this patent, a metal cation salt/polymer gelis formed by mixing an aqueous continuous phase with a hydrophilicorganic polymeric disperse phase. When the hydrophilic organic polymeris added to the solution, the hydrophilic organic polymer absorbs theliquid on to its structure due to chemical affinity. The product is agel with the metal salt solution “frozen” within the dispersed polymericnetwork. The salt/polymer network is calcined to decompose the powder,leaving a high surface metal oxide powder. The calcining temperature isstated to be from 300° C. to 1,000° C., preferably 450° C. to 750° C.

This patent requires that a hydrophilic organic polymer be used in theprocess for making metal oxide powders.

Other patents that describe the production of nanometre-sized powdersinclude U.S. Pat. No. 5,338,834 (incorporate a metal salt solution intoa polymeric foam and calcining the foam to remove organics and leave apowder) and U.S. Pat. No. 5,093,289 (a foam matrix is coated with asuspension of silicon powder, synthetic resin and solvent and is subjectto a heat treatment during which the foam is expelled and the silicon isstabilized).

The present inventors have now developed a method for producingparticles, especially metal oxide particles.

In one aspect, the present invention provides a method of producingparticles having nano-sized grains, the method comprising the steps of:

(a) preparing a solution containing one or more metal cations;

(b) mixing the solution from step (a) with one or more surfactant underconditions such that micelles are formed, and

(c) heating the mixture from step (b) above to form the particles .

Preferably, the particles are metal oxide particles and step (c) formsparticles of metal oxide.

The particles are preferably agglomerates of the grains. In thisembodiment, the grains are suitably lightly sintered together.

The method may optionally further comprise the steps of treating themixture from step (b) to form a gel and heating the gel to form theparticles of metal oxide.

Step (a) of the present process involves the preparation of a solutioncontaining one or more metal cations. The metal cations are chosenaccording to the required composition of the metal oxide particles. Thesolution of one or more metal cations is preferably a concentratedsolution. The inventors presently believe that a high concentration ofdissolved metal is preferred for achieving the highest yield of product.

A very large number of metal cations may be used in the presentinvention. Examples include metal cation from Groups 1A, 2A, 3A, 4A, 5Aand 6A of the Periodic Table, transition metals, lanthanides andactinides, and mixtures thereof. This list should not be considered tobe exhaustive. The mixture may contain one or more different metalcations.

The metal cation solution is suitably produced by mixing a salt or saltscontaining the desired metal(s) with a solvent. Any salt soluble in theparticular solvent may be used. The metal cation solution may also beproduced by mixing a metal oxide or metal oxides or a metal or metalswith appropriate solvent(s).

A number of solvents can be used to prepare the metal cation solution.The solvents are preferably aqueous-based solvents. Examples of suitablesolvents include nitric acid, hydrochloric acid, sulphuric acid,hydrofluoric acid, ammonia, alcohols, and mixtures thereof. This listshould not be considered exhaustive and the present invention should beconsidered to encompass the use of all suitable solvents.

Step (b) of the method of the present invention involves addingsurfactant to the mixture to form micelles. Preferably, the surfactantis added to the solution such that a micellar liquid is formed.

A micellar liquid is formed when surfactant is added in sufficientquantity such that the surfactant molecules aggregate to form micelles.Use of micellar liquid enables simple, rapid and thorough mixing of thesolution and surfactant, which is important for commercial productionprocesses. It is preferred that the amount of surfactant mixed with thesolution is sufficient to produce a micellar solution in which themicelles are closely spaced.

The conditions under which the micellar liquid is formed will dependupon the particular surfactant(s) being used. In practice, the mainvariables that need to be controlled are the amount of surfactant addedand the temperature. For some surfactants, the temperature should beelevated, whilst for others room temperature or below is necessary.

Any surfactant capable of formning micelles may be used in the presentinvention. A large number of surfactants may be used in the invention,inlcuding non-ionic sufactants, cationic sufactants, anionic surfactantsand zwitteronic surfactants. Some examples include BrijC₁₆H₃₃(OCH₂CH₂)₂OH, designated C₁₆EO₂, (Aldrich); Brij 30, C₁₂EO₄,(Aldrich); Brij 56, C₁₆EO₁₀, (Aldrich); Brij 58, C₁₆EO₂₀, (Aldrich);Brij 76, C₁₈EO₁₀, (Aldrich); Brij 78, C₁₆EO₂₀, (Aldrich); Brij 97,C₁₈H₃₅EO₁₀, (Aldrich); Brij 35, C₁₂EO₂₃, (Aldrich); Triton X-100,CH₃C(CH₃)₂CH₂C(CH₃)₂C₆H₄(OCH₂CH₂)_(x)OH,x=10(av), (Aldrich); TritonX-114, CH₃C(CH₃)₂CH₂C(CH₃)₂C₆H₄(OCH)₂CH₂)₅OH (Aldrich); Tween 20,poly(ethylene oxide) (20) sorbitan monokayrate (Aldrich); Tween 40,poly(ethylene oxide) (20) sorbitan monopalmitate (Aldrich); Tween 60,poly(ethylene oxide)(20) sorbitan monostearate (Aldrich); Tween,poly(ethylene oxide) (20) sorbitan monooleate (Aldrich); and Span 40,sorbitan monopalmitate (Aldrich), Terital TMN 6,CH₃CH(CH₃)CH(CH₃)CH₂CH₂CH(CH₃)(OCH₂CH₂)₆OH (Fulka); Tergital TMN 10,CH₃CH(CH₃)CH(CH₃)CH₂CH₂CH(CH₃)(OCH₂CH₂)₁₀OH (Fulka); block copolymershaving a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)(EO-PO-EO) sequence centered on a (hydrphobic) poly(propylene glycol)nucleus terminated by two primary hydroxyl groups; Pluronic L121(_(Mav)=4400), EO₅ ₅PO₇₀EO₅ (BASF); Pluronic L64 (_(Mav)=2900),EP₁₃PO₃₀EO₁₃ (BASF); Pluronic P65 (_(Mav)=3400), EP₂₀PO₃₀EO₂₀(BASF);Pluronic P85 (_(Mav)=4600), EO₂₆PO₃₉EO₂₆ (BASF); Pluronic P103(_(Mav)=4950), EO₁₇PO₅₆EO₁₇ (BASF); Pluronic P123 (_(Mav)=5800),EO₂₀PO₇₀EO₂₀, (Aldrich); Pluronic F68 (_(Mav)=8400), EO₈₀PO₃₀EO₈₀(BASF); Pluronic F127 (_(Mav)=12 600), EO₁₀₆PO₇₀EO₁₀₆ (BASF); PluronicF88 (_(Mav)=11 400),EO₁₀₀PO₃₉EO₁₀₀(BASF); Pluronic 25R4 (_(Mav)=3600),PO₁₉EO₃₃PO₁₉ (BASF); star diblock copolymers having four EO_(n)-PO_(m)chains (or in reverse, the four PO_(n)-EO_(m) chains) attached to anethlenediamine nucleus, and terminated by secondary hydroxyl groups;Tetronic 908 (_(Mav)=25 000), (EO₁₁₃PO₂₂)₂NCH₂CH₂N(PO₁₁₃EO₂₂)₂ (BASF);Tetronic 901 (_(Mav)=4700), (EO₃PO₁₈)₂NCH₂CH₂N(PO₁₈EO₃)₂ (BASF); andTetronic 90R4 (_(Mav)=7240), (PO₁₉EO₁₆)₂ NCH₂CH₂N(EO₁₆PO₁₉)₂ (BASF)

The above sufactants are non-ionic surfactants. Other surfactants thatcan be used include:

Anionic Surfactant:

Sodium dodecyl sulfate CH₃(CH₂)₁₁OSO₃NA

There appears to be several manufacturers. Sigma is an example.

Cationic Surfactants:

Cetyltrimethylammonium chloride CH₃(CH₂)₁₅N(CH₃)₃C1 Aldrich

Cetyltrimethylammonium bromide CH₃(CH₂)₁₅N(CH₃)₃BT Aldrich

Cetylpyridinium chloride C₂₁H₃₈NC1 Sigma.

This list should not be considered to be exhaustive.

Step (c) of the method of the present invention involves heating of themixture from step (b) to an elevated temperature to thereby form themetal oxide particles. This step may optionally be preceded by a step oftreating a solution to form a gel. Typically, it is sufficient to changethe temperature of the mixture to form the gel. For some mixtures,cooling will result in gel formation. For other mixtures, heating willresult in gel formation. This appears to be dependent upon thesurfactant(s) used.

If the optional step of forming a gel is used in the method, the heatingof step (c) involves heating the gel.

The heating step results in the formation of the metal oxide and thepore structure of the particles. Unlike prior art processes forproducing metal oxides, the method of the present invention onlyrequires a relatively low applied temperature. Indeed, appliedtemperatures of less than about 300° C. have been found to be suitablein experimental work conducted to date. Preferably, the maximum appliedtemperature reached in step (c) does not exceed about 600° C., morepreferably about 450° C., most preferably about 300° C. The presentinventors believe that the process of the present invention may involvelocalised exothermic reactions occurring, which could lead to highlylocalised temperatures. However, it remains a significant advantage ofthe present invention that the applied temperature is relatively lowcompared to prior art processes known to the inventors.

The heating step may involve a rapid heating to the maximum desiredtemperature, or it may involve a much more closely controlled heattreatment regime. For example, the heating step may involve heating to adrying temperature (generally below the boiling temperature of themixture) to dry the mixture, following by a slow ramp up to the maximumapplied temperature, or followed by a series of incremental increases tointermediate temperatures before ultimately reaching the maximum appliedtemperature. The duration of the heating step may vary widely, with apreferred time in step (c) being from 15 minutes to 24 hours, morepreferably 15 minutes to 2 hours even more preferably 15 minutes to 1hour. It will be appreciated that step (c) is intended to encompass allheating profiles that result in the formation of particles of metaloxide.

The metal oxide particles produced by preferred embodiments of themethod have nano-sized grains. Preferably, the grain size falls withinthe range of 1-50 nm, more preferably 1-20 nm, even more preferably 2-10nm, most preferably 2-8 nm.

The grain size was determined by examining a sample of the particlesusing TEM (transmission electron microscopy), visually evaluating thegrain size and calculating an average grain size therefrom. Theparticles may have varying particle size due to the very fine grainsaggregating or cohering together. The particle size may vary from thenanometre range up to the micrometre range or even larger. The particlesmay have large specific surface areas (for the particular metal oxide,when compared with prior art processes for making those particles) andexhibit a broad distribution of pore sizes.

The present invention also encompasses metal oxide particles. In asecond aspect, the present invention provides metal oxide particlescharacterised in that the particles have a grain size substantially inthe range from 1 to 50 nm.

Preferably, the grain size falls within the range of 1 to 20 nm morepreferably 2 nm to 10 nm, more preferably 2 nm to 8 nm.

The particles are preferably substantially crystalline and contain onlysmall or negligible amounts of amorphous material.

The particles preferably have other properties as described withreference to the particles described with reference to the first aspectof the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF INVENTION AND EXAMPLES

Preferred embodiments of the present invention involve the followingsteps:

-   -   (a) preparation of a concentrated aqueous solution containing        metal cations of at least one metal (by “concentrated solution”,        it is meant that the metal cations are present in an amount of        90% or greater of the theoretical solubility limit in the        particular solvent/solute system utilised);    -   (b) creation of a micellar liquid—the solution from step (a) is        mixed with a surfactant at a temperature where the mixture forms        a micellar liquid;    -   (c) (optional) formation of a gel—the temperature of the        micellar liquid is altered to form a gel. The gel forms due to        ordering of surfactant molecules or surfactant micelles; and    -   (d) heat treatment—the heat treatment forms the metal oxides,        removes all surfactant and creates the pore structure.

EXAMPLE 1 Production of CeO₂

In order to demonstrate the method of the present invention, particlesof CeO₂ were produced. The following procedure was used:

Step 1: A cerium nitrate solution containing 2.5 moles/litre ceriumnitrate was prepared.

Step 2: 16 g Brij 56 surfactant and 20 mls cerium nitrate solution wereheated to ˜80° C. At this temperature the surfactant is a liquid. Thesolution was added slowly to the surfactant liquid while stirring, tocreate a micellar liquid.

Step 3: The micellar liquid was cooled to room temperature. During thecooling the liquid transformed to a clear gel.

Step 4: The gel was heat treated according to temperature historypresented in FIG. 4. In this example, an extended drying stage at 83° C.was used prior to further heating.

The resulting CeO₂ powder had a surface area of ˜253m²/g, and wascomprised of grains that ranged between ˜2 and ˜8 nm in diameter.Transmission electron microscopy (TEM) suggests that the final powderconsisted of lightly sintered aggregates of very fine grains. This isshown schematically in FIG. 5, and a TEM photomicrograph of the productis shown as FIG. 10.

FURTHER EXAMPLES

Several target metal oxide materials were chosen to test thecapabilities of the process. Some of these materials aremulti-component, complex metal oxides that are very difficult to formusing conventional methods.

Ceria-based Compounds

CeO₂, and other mixed oxides containing cerium and one or more ofsamarium, copper and zirconium Ce_(0.6)Sm_(0.4)O_(x),Ce_(0.65)Sm_(0.2)Cu_(0.15)O_(x), andCeo_(0.6)Zr_(0.2)Sm_(0.1)Cu_(0.1)O_(x) have been produced. The oxygencontent is represented by x since the exact content is dependent uponcomposition and is not precisely known at this stage. These materialsare excellent candidates for catalytic applications, and may also beused on SOFC anodes. They are also a very useful test of the ability ofthe present invention to produce multicomponent oxides. All of thesecompositions should exhibit the basic crystal structure of CeO₂ if thedifferent metal components are evenly distributed throughout thematerial. This is because the additional elements can be incorporatedinto the CeO₂ crystal structure. However, inhomogeneous distribution ofelements may result in pockets of material that may have much higherconcentrations of one or more particular elements. Such pockets can formdifferent crystal structures (or phases).

X-ray diffraction has been used to determine whether the materials aresingle-phase CeO₂ crystal structure (evenly distributed elements), orcontain additional crystal structures that would indicate poor mixing ofelements. The surface areas and grain sizes of several materials havealso been measured.

FIG. 6 shows XRD traces from CeO₂, Ce_(0.6)Sm_(0.4)O_(x),Ce_(0.65)SM_(0.2)Cu_(0.15)O_(x), and Ce0.6Zr_(0.2)Sm_(0.1)Cu_(0.1)O_(x)that were made using our process. The XRD traces showed that the correctCeO₂ crystal structure was obtained in all materials, even the fourcomponent system. This strongly suggests a very uniform distribution ofelements. The width of the peaks indicates that the grain size isextremely small in all these materials.

The surface areas obtained for CeO₂, Ce_(0.6)Sm_(0.4)O_(x),Ce_(0.65)Sm_(0.2)Cu_(0.15)O_(x), using Brij 56 surfactant andnon-optimal heat treatments, were 219, 145 and 171m²/g, respectively.Ce0.6Sm_(0.4)O_(x) and Ce_(0.65)Sm_(0.2)Cu_(0.15)O_(x) powders were abrowny yellow colour from the Sm and Cu. They were held at 300° C. forlonger than the CeO₂ to ensure that all surfactant was removed (withCeO₂, surfactant removal can be clearly observed via a change in colourfrom brown to yellow). This longer time at 300° C. was probablyresponsible for the lower surface areas in these materials, compared toCeO₂.

The pore structure of the CeO₂ material, and its relationship tosurfactant order in the gels, was further investigated. FIG. 7 showssmall angle x-ray scattering (SAXS) data for gels comprised of ceriumnitrate solutions and Brij 35, Brig 56 and Pluronic F127 surfactants.Also shown are SAXS data for the powders produced from these gels.Significant peaks on the data from all three gels indicate the presenceof ordered surfactant structures. This order is clearly not present inthe final powders.

Nitrogen adsorption was used to determine the pore-size distribution(FIG. 8). The distribution is very broad, indicating that the porestructure did not result simply by pores replacing surfactant micelles.The results are compared to the pore size distribution obtained by Zhaoet. al. (J. Am. Chem. Soc. vol. 120 (1998) p6024-6036) forsurfactant-templated silica (using the same surfactant) in FIG. 9. Thetotal pore volumes are similar when the different densities of silicaand CeO₂ are taken into account. However, the pore size distribution isclearly much broader in the CeO₂ material. This indicates that the poresin the CeO₂ were not created simply by occupying the same space as thesurfactant micelles, in contrast to surfactant-templated materials.

Transmission electron microscopy (TEM) of the CeO₂ material shows thatthe grain size is extremely small. The grains range between ˜2 nm and ˜6nm in diameter (see the TEM micrograph print of FIG. 10). This is closeto the limiting grain size, which is determined by the atomic ‘unitcell’ of a material. Typically, unit cell dimensions for metal oxidesrange between 1 and 2 nm.

EXAMPLE Preparation of La_(0.6)Ca_(0.2)Nd_(0.2)Mn_(0.9)Ni_(0.1)O₃

La_(0.6)Ca_(0.2)Nd_(0.2)Mn_(0.9)Ni_(0.1)O₃ is used as the cathodematerial in solid oxide fuel cells. It is also an excellent testmaterial for the present invention because the target ‘lanthanummanganate’ crystal structure is extremely sensitive to chemicalcomposition. Even small variations in composition result in theformation of different crystal structures. Therefore, the five differentmetal elements need to be evenly distributed on an extremely fine scaleto produce small grains with the correct crystal structure.

Using co-precipitation and other conventional processes, previousresearchers have had considerable difficulty in obtaining the correctcrystal structure because of this sensitivity to composition. Carefulco-precipitation, followed by long (10h-48h) heat treatments at hightemperatures (800-1000° C.) have been necessary to attain the correctcrystal structure in the prior art (variations in chemical compositioncan be alleviated by diffusion of atomic elements at these hightemperatures). One result of this high temperature processing is thatsignificant grain growth and sintering of grains occurs so that thesurface areas obtained are very low and grain size is relatively large.

FIG. 11 shows an XRD trace fromLa_(0.6)Ca_(0.2)Nd_(0.2)Mn_(0.9)Ni_(0.1)O₃ material produced using themethod of the present invention. A Pluronic F127- metal nitrate solutiongel was used, and the heat treatment consisted of 1 hour at 100° C.,followed by 0.5 hour at 300° C. The trace indicates that the material isthe targeted lanthanum manganate crystal structure. This is an amazingresult given the very low temperatures used for heat treatment. Asurface area of ˜30m²/g was obtained for this material. While 30m²/g ismuch lower than the values for CeO₂-based materials, it is consideredvery high for this material. Recently, a surface area of 55m²/g wasachieved from the present method using metal acetate solutions insteadof metal nitrates, indicating that significant improvements may yet beachieved. This result also indicates that use of different salts, ienitrates, acetates, etc, may give different surface area results.

SAXS data for this material is shown in FIG. 12. As for the CeO₂materials, there are no peaks on the SAXS data, indicating a lack oforder in the pore structure.

The experimental work conducted to date by the present inventors hasused metal cation solutions having a high concentration of dissolvedmetal. Experiments conducted to date have used metal salt solutions thatare close to the solubility limits in order to attain the best yield.However, it is to be understood that the present invention should not beconsidered to be limited to using concentrated solutions of metalcations.

Experiments in Step 2: Mixing the Solution with Surfactant

Four different types of surfactant have been trialed: Brij 30, Brij 35,Brij 56 and Pluronic F127. The Brij surfactants are mixed at hightemperatures where they form micellar liquids with aqueous solutions,and can be cooled to form gels. With these surfactants it is possible toheat-treat straight from the micellar liquid stage without forming agel. In contrast, Pluronic surfactants form micellar liquids in aqueoussolutions at low temperatures (˜0° C.) and form gels upon heating. It istherefore not possible to heat-treat Pluronic F127 mixtures withoutfirst forming a gel.

For CeO₂ materials, Brij 30, Brij 35 and Brij 56 surfactants producedmuch higher surface areas (>200m²/g) than Pluronic F127 surfactant(˜30m²/g). The inventors are unsure of the reason for this. It appearsthat Brij 56 may produce higher surface areas than Brij 35 however moreinvestigations using a range of heat treatments are needed to confirmthis.

For La_(0.6)Ca_(0.2)Nd_(0.2)Mn_(0.9)Ni_(0.1)O₃ material, the situationwas reversed. Using metal nitrate solutions, Pluronic F127 resulted in asurface area of ˜30m²/g, while the Brij surfactants yielded <10m²/g.

Although the reasons for achieving differences in surface areas are notyet understood, the present invention does appear to provide the abilityto produce materials with different surface areas. This may be a furtheradvantage of the present invention. For example, for many metal oxideapplications, it is necessary to manufacture a solid ceramic device withminimal porosity (eg the solid electrolyte in solid oxide fuel cells).In these applications, a high surface area is not important or evendesirable. However, fine grains can still be advantageous since theyreduce sintering temperatures and may deliver improved physicalproperties. It appears that the method of the present invention can betailored to suit these applications, as well as applications thatrequire porous, high surface area materials.

The present inventors also believe that the concentration of surfactantwill certainly affect the resultant materials produced by the method ofthe present invention. As yet, no experimental work confirming this hasbeen conducted.

The heat treatment step of the present invention sees the metal oxidesand the pore structures both being formed during this stage.

In the experiments conducted by the present inventors to date, whichmainly related to the production of metal oxides from nitrate solutions,the inventors have postulated that a high density of finely spacedmicelles present in the micellar liquid probably hinders growth ofprecipitates, which may explain the very small grain sizes that havebeen obtained. The confined spaces between micelles may also prevent anylarge scale separation of different metal elements. It is believed thatthe metal nitrates decompose, as evidenced by emissions of nitrous oxide(Nox) gases. It is believed that the latter stages of the heat treatmentinvolve a combustion reaction, which may burn at least part of thesurfactant out of the product.

It will be realised that the above mechanism is only a postulatedmechanism and the present invention should not be construed as beinglimited to that particular mechanism.

The present inventors are also unsure as to the mechanisms that lead tothe high surface area or pore structures being formed. The very broadpore size distributions show that the pores are not simply created inspaces that were occupied by the micelles. The present inventors believethat it is possible that the segregation of liquid and precipitatednitrates into confined spaces between micelles, and gases released fromnitrate decomposition and/or surfactant decomposition, combine to formthe high surface area of pore structures. Again, the present inventorshave only postulated this mechanism and the present invention should notbe construed as being limited to this particular mechanism.

A range of heat treatments were applied to cerium nitratesolution/surfactant gel to try to gain some understanding of how variousheat treatment parameters affect the surface areas of the final powders.These heat treatment regimes are shown in FIG. 13.

Heat treatment no. 1 was designed to produce a dried gel, and to combustthis dried gel extremely rapidly. Heat treatment no. 2 again produced adried gel, however the combustion was designed to be much morecontrolled than for heat treatment no. 1. Heat treatment no. 3 did notproduce a dried gel prior to further heating and was the simplest andquickest of the three heat treatments. It is therefore particularlyattractive as a commercial process.

In heat treatment no. 1, during the long, low temperature stage, the geldried into a hard, yellow mass. A significant number of bubbles evolvedand were trapped in the mass at this stage. When placed upon a hot plateat 300° C., the dried gel ignited immediately and violently to form ayellow powder. The powder was cerium oxide with a surface area of170m2/g.

With heat treatment no. 2, the dried gel softened and partly turned toliquid at about 100° C., then significant NO_(x), gas was released, andfinally a slow combustion reaction occurred. The combustion reaction wasevidenced by a slowly moving red front. A browny-yellow powder waspresent after combustion. Over time at approximately 200° C., thispowder turned more yellow, probably due to burn-off of residualsurfactant. The final cerium oxide powder had a surface area of 253m²/g.

Heat treatment no. 3—the gel turned to liquid shortly after placement onthe hot plate. Evaporation of water and emission of NO_(x), gasfollowed. A grey-brown-yellow mass result. Finally, a slow combustionreaction again proceeded along a red front, turning the mixture blackthen browny-yellow. Over time at approximately 300° C., this powderturned more yellow, probably due to bum-off of residual surfactant. Thisheat treatment produced cerium oxide powder with the surface area of219m²/g.

These experiments clearly showed the importance of heat treatment indetermining the final properties of the powders. Very rapid combustionresulted in the lowest surface area. Slower heating and combustion of adried gel resulted in the highest surface area. Simply placing a wet gelon the hot plate also produced a very high surface area. These generaltrends were also observed in other experiments with differentsurfactants and different materials.

The present invention provides the following advantages over the priorart known to the present inventors:

(a) the metal oxides produced have extremely small grain sizes. Forexample, cerium dioxide materials have grain sizes ranging between about2 and about 10 nanometres;

(b) the metal oxides produced are highly crystalline, ie they have ahigh degree of atomic order. This is an important advantage over mostsurfactant-templated materials, which have almost no atomiccrystallinity;

(c) extremely high surface areas may be obtained for some metal oxides(compared to prior art processes). The surface areas of the resultantpowders are dependent upon the type of surfactant used, the type ofmetal ions, and the heat treatment. It also appears that the type ofsalt (eg nitrate, acetate, chloride, etc) may influence the surfacearea;

(d) very complex, multi-component metal oxides can be produced using thepresent invention. This indicates that different atomic species areevenly distributed throughout the material;

(e) low applied temperatures (less than about 300° C.) are sufficient toform even multi-component metal oxides. Indeed, the present inventorshave literally conducted the majority of their experiments to date on ahot plate. This is a major advantage over other techniques, particularlyfor the production of multi-component metal oxides, which normallyrequire heat treatments at high applied temperatures (approximately1,000° C.) for extended periods to obtain the correct metal oxide phase.In particular, this has apparent benefits in reduced capital costs forfurnaces, reduced operating expenses and avoiding undesirable sinteringand grain growth that would occur at the high temperatures.

(f) the process is extremely rapid. The inorganic reaction and entireheat treatment may be done in as little as 30 minutes. This compareswith conventional techniques that require long heat treatments (in somecases, up to several days). The long inorganic reactions that arecharacteristic of surfactant-templating methods are not used andtherefore the present invention is much quicker thansurfactant-templating processes;

(g) the process uses low cost raw materials and simple processingtechnology. It is therefore extremely inexpensive;

(h) in cases where heating of a gel is conducted, the gels consist ofordered surfactant structures. However, this ordered structure isdefinitely not present in the final materials. In addition, pore sizedistributions are very broad, indicating that the pores do not resultfrom simple burn-out of surfactant micelles. The pore structure istherefore significantly different to that in the surfactant-templatedmaterials described previously.

Those skilled in the art will appreciate that the present invention maybe susceptible to variations and modifications other than thosespecifically described. It is to be understood that the presentinvention encompasses all such variations and modifications that fallwithin its spirit and scope.

1-20. (Canceled)
 21. Particles of mixed metal oxide that include atleast two metal species, said particles having a grain size within therange of 1-100 nm, wherein the particles are substantially crystallineand contain only small or negligible amounts of amorphous material andwhere the at least to metal species are uniformly dispersed in theparticles.
 22. Particles according to claim 21, wherein the particleshave disordered pore structures.
 23. Particles according to claim 21,wherein the particles exhibit a broad distribution of pore sizes. 24.Particles according to claim 21, wherein the metal oxide is formed froma metal cation selected from the group consisting of metal cations fromGroups 1A, 2A, 3A, 4A, 5A and 6A of the Periodic Table, transitionmetals, lanthanides and actinides, and mixtures thereof.
 25. Particlesaccording to claim 21, wherein the grain size of the particles fallswithin the range of 1-50 nm.
 26. Particles according to claim 25,wherein the grain size of the particles falls within the range of 1-20mn.
 27. Particles according to claim 25, wherein the grain size of theparticles falls within the range of 1-10 nm.
 28. Particles according toclaim 25, wherein the grain size of the particles falls within the rangeof 2-8 nm.
 29. Particles according to claim 21, wherein the particlesare phase-pure.